[Schroedingersat]

Sodium acetate has a heat of fusion of 289kJ/kg and a price of around $400/tonne

For low grade heat you can just store it directly using some black pipes at $5/kWh of storage skipping the electrolysis and the pv steps.

Might need some kind of cleaning after 5 years, but it should break even especially if scale pushes prices down a bit.